When I go car shopping, I carry my OBD2 scanner with me, and if I'm interested, I plug it in and check for codes as well as the system status for emmissions and such. I also make a deal out of writing down the VIN to run a car fax. I had one used car dealer that wouldn't let me use the scanner, and I walked away, the scanner will tell you if the CEL has a "malfunction".

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2000 C230 Kompressor "Frosty"

2007 PT Cruiser
2001 VW TDI Beetle
1999 Landrover Discovery II
1971 Corvette

Gone: 97 Cabrio, 83 240D, 93 VW Eurovan, 95 Volvo 850, 93 Chevy Astro Van, 77 VW Bus, 99 VW Jetta, 74 VW Bus